The formation of thin layers from a donor substrate is a conventional step in microtechnology. Among the main techniques for such formation, the method known by the name “Smart Cut™” may be mentioned.
The method “Smart Cut™” makes it possible to detach a thin film and transfer it onto a stiffener by the following steps:                1. bombarding a face of a donor substrate with gas species (H or a rare gas) in order to implant these ions in a sufficient concentration to cause the creation of a buried layer weakened by the presence of microcavities,        2. placing this face of the substrate in intimate contact with a stiffener and        3. fracturing the layer of microcavities by applying a heat treatment and/or a detachment stress (for example inserting a blade between the two substrates and/or traction and/or flexion and/or shear forces, and/or the application of ultrasound or microwaves with an expediently selected power and frequency).        
Conventionally, in order to form a plurality of thin layers from such a donor substrate, the remainder of the substrate obtained at the end of a cycle is recycled by using this remainder as the donor substrate for a new cycle of steps 1-3 above. Thus, using the same donor substrate, a plurality of thin films are transferred onto different stiffeners.
In practice, the thickness of the thin layer (the term thin film is sometimes used) is so small, typically less than a few microns, that its detachment from the donor substrate resembles peeling.
In fact, the presence of a stiffener is not always required; in particular, a stiffener may be omitted when the thickness of the thin layer is sufficient for it to be self-supporting. In other words, the presence of a stiffener serves mainly to facilitate the handling of a thin layer which has just been detached from a donor substrate.
However, with the trend to constantly reduce the thickness of thin layers, the presence of a handle substrate (used for stiffening or handling) is more and more desirable.
It will be understood that the remainder obtained by applying an aforementioned cycle to a donor substrate can be recycled only on condition that the surface of the face uncovered during the preceding cycle has a sufficient quality to allow it to be fixed to a new handle substrate.
For various reasons, however, the step of fracture along the buried weakened layer leads to detachment of only a part of the thin film; this is because the film locally remains integral with the donor substrate in zones referred to as non-transferred zones, thus forming elevations with a thickness varying between 10 and 1000 nm (in general substantially equal to the thickness of the thin film). Recycling of the donor substrate then requires particular steps of planarization by mechanical polishing and/or chemical attack.
The existence of such non-transferred zones is due to the fact that, ideally, step 2 of placing the donor substrate (previously implanted) in intimate contact with the stiffener should be carried out on plane and perfectly clean surfaces. The problem is that, when carrying it out, this intimate contacting cannot be achieved over the entire surface of the substrates:                1) first, the donor substrate (as well as the handle substrates forming stiffeners) are typically wafers whose edges are chamfered. This problem of wafer edge non-bonding is encountered for all the materials used in microtechnology (Si, Ge, GaAs, GaN, sapphire, SiGe, LiTaO3, LiNbO3, SiC, InP, and the like) and for all wafer diameters (typically from 5 cm to 30 cm, in particular from 2 to 12 inches),        2) furthermore, in the case of substrates or layers which are structured intentionally (for example with hollow/relief patterns produced by photolithography) or unintentionally (for example by growth defects in the case of epitaxial layers, or by defects associated with the deposition of a layer on the initial substrate), the patterns or defects “hollowed” on the surface locally give rise to non-bonded zones (abbreviated to NBZ),        3) lastly, in the event of ineffective cleaning, the presence of particles on the bonding surface also gives rise to non-bonded zones.        
During the fracture step, when the dimension of the NBZs is large compared with the thickness of the film to be transferred (for example a ratio [lateral NBZ dimension]/[film thickness] of the order of 10), the thin film locally remains integral with the initially implanted substrate. These are referred to as non-transferred zones (abbreviated to NTZ).
FIG. 1 schematically represents a donor substrate 1 which is attached by molecular adhesion to a handle substrate 2.
The bonding of the two substrates 1 and 2 is carried out by means of bonding layers 11 and 21 formed on the free faces of the two substrates.
These two substrates (here “wafers”) are chamfered at their peripheries denoted 12 and 22. Furthermore, as a consequence of cleaning which has not been completely effective, a particle 3 is trapped in the interface S between the bonding layers so that, at the position of this particle, the two substrates are not bonded to one another (of course, depending on the materials selected for the two substrates, it is possible to omit the bonding layers); in the example considered, the two substrates are made of silicon and the bonding layers are made of silicon oxide (this may involve the thermal oxide layer naturally present on the surface of the substrate, but it may also involve a thicker layer formed intentionally).
The donor substrate has been weakened beforehand by implanting gas species in a layer 13 buried at a depth, under the surface belonging to the interface, which determines the thickness of the future thin layer which is intended to be transferred from the donor substrate to the handle substrate; the reference 14 denotes the layer lying between the buried layer and the bonding interface, detachment of which will give the thin layer.
FIG. 2 schematizes the fact that, owing to the lack of bonding at the edges of the substrates (because they are chamfered) and at the position of the particle, the layer 14 is not detached everywhere from the donor substrate 1: there remain edge portions A and A′ and an island B, corresponding to the position of the particle. In these FIGS. 1 and 2, of course, the thickness of the layer 14 is highly exaggerated in comparison with the dimensions of the particles or chamfered edges.
In practice, these non-bonded zones lead to very local lifting or detachment (over dimensions of a few μm2) of the thin layer, in the form of blisters or exfoliated zones.
The dimensions of the non-transferred zones depend not only on the dimensions of the non-bonded zones but also on the adhesion force (this force depends in particular on the properties of the surfaces placed in contact, but also on the operating conditions of a possible heat treatment during or after the implantation).
FIG. 2 clearly demonstrates the need, for recycling, to provide a treatment of the surface of the substrate which has been uncovered by detaching the layer 14 in order to give the thin layer 4, this treatment aiming to planarize this surface in order to eliminate the elevations formed by the non-transferred zones from it.
Examples of planarization steps are described particularly in the published European Patent Application Nos. EP1427002 and EP1427001 in relation to the periphery of the substrates where a neck remains, consisting of the peripheral part of the layer which has to form the thin layer. It is thus recommended, in particular, to apply a planarization treatment localized at the periphery, comprising polishing or the application of a mechanical pressure, or a selective chemical attack or an ion attack (for example by an ion beam). As a variant, it is recommended to separate the neck by spraying a jet of fluid onto this neck, or by projecting a laser beam onto this neck, preferably parallel to the exposed surface, or by applying a shock wave to the rear face of the substrate. This treatment localized at the periphery may be followed by a planarization treatment applied to the entire surface.
It will, however, be understood that such localized treatments may be complex to carry out.